On the population of primordial star clusters in the presence of ultraviolet background radiation

We use the algorithm of Cole et al. to generate merger trees for the first star clusters in a cold dark matter ( CDM) cosmology under an isotropic ultraviolet background radiation field, parametrized by J21. We have investigated the problem in two ways: a global radiation background and local radiative feedback surrounding the first star clusters. Cooling in the first haloes at high redshift is dominated by molecular hydrogen, H2 – we call these Generation 1 objects. At lower redshift and higher virial temperature, T vir 10 4 K, electron cooling dominates – we call these Generation 2. Radiation fields act to photodissociate H2, but also generate free electrons that can help to catalyze its production. At modest radiation levels, J 21/(1 + z) ∼ 10−12–10−7, the nett effect is to enhance the formation of Generation 1 star clusters. At higher fluxes, the heating from photoionization dominates and halts their production. With a realistic build-up of flux over time, the period of enhanced H2 cooling is so fleeting as to be barely discernable and the nett effect is to move primordial star cluster formation towards Generation 2 objects at lower